I want to serialize JSON from array to object

public class Atlas_Purchase
    {
        public string ReferId { get; set; }
        public List<string> DestinationList { get; set; }
        public List<AtlasApplicantList> ApplicantList { get; set; }
        private List<AtlasCreditCardDetails> CreditCards = new List<AtlasCreditCardDetails>();
        public List<AtlasCreditCardDetails> CreditCard
        {
            get { return CreditCards; }
            set { CreditCards = value; }
        }

public class AtlasApplicantList
    {
      public string FirstName { get; set; }
        public string LastName { get; set; }
        public string Dob { get; set; }
        public string Gender { get; set; }
}

public class AtlasCreditCardDetails
    {
        public string CardExpirationMonth { get; set; }
        public string CardExpirationYear { get; set; }
        public string CardHolderAddress1 { get; set; }
        private List<AtlasTransaction> Transactions = new List<AtlasTransaction>();
        public List<AtlasTransaction> Transaction
        {
            get { return Transactions; }
            set { Transactions = value; }
        }
    }

JavaScriptSerializer js = new JavaScriptSerializer();
string jsonData = js.Serialize(purchase);

String jsonData = JsonConvert.SerializeObject(purchase);